In recent years, in many cases, wiring boards are used in high-frequency regions having a frequency band in the order of GHz or more, with an increase in speed of information communication. Thus, in order to reduce a transfer loss of electric signals, the wiring boards are desired to use a metal having low conductor resistance, such as Ag and Cu, as a conductor layer thereof, thereby reducing a conductor loss. For this reason, low temperature-sintering materials that can be sintered simultaneously with a low-melting metal such as Ag and Cu and that have superior high-frequency dielectric characteristics are being compared and investigated.
Now, there are known crystallized glasses as a low temperature-sintering material having superior high-frequency characteristics, in which crystals having superior high-frequency characteristics crystallize during the sintering step.
As these crystallized glasses, there are, for example, known (1) crystallized glass of a spinel type crystal (for example, JP-A-9-175853(The term “JP-A” as used herein means an “unexamined published Japanese patent application”)), (2) crystallized glass of diopside (for example, JP-A-10-120436), and (3) crystallized glass of alumina and anorthite (for example, JP-A-2000-143332).
In addition, multilayered wiring boards that can realize relatively high-density wiring are widely used as wiring boards mounted with a semiconductor element such as LSI, IC, and discrete parts, or having a varied thick-film printing element incorporated therein. Most of these multilayered wiring boards are an alternate laminate of a dielectric layer comprising a glass ceramic and a conductor layer mainly composed of a metal conductor such as Cu, Ag, Au, W, and Mo, and if desired, a semiconductor element is mounted on the surface thereof.
Further, in recent years, in radiocommunication including portable telephone, in order to enlarge the radio resource and realize high density of the transmission volume, there have been positively employed high-frequency bands from micro-wave bands to millimeter wave bands. As parts for radiocommunication instruments to be used, demands of multilayered wiring boards for dealing with high-frequency signals are explosively increasing.
In the case of dealing with the high-frequency signals, since a conductor layer connecting a working electric source of an electronic part to the electronic part contributes as an inductance, there may possibly occur inconveniences that are not problematic in low-frequency signals, such as generation of malfunction caused by superimposition of noises on wirings in the conductor layer, delay of operation response of electronic parts, and transmission loss of high-frequency signals. In order to inhibit such inconveniences inherent to the high-frequency signals, it is required to use multilayered wiring boards in which the conductor layer is constituted of a low-resistivity material, whereas the dielectric layer is constituted of a material having a low specific dielectric constant and a low dielectric loss in a high-frequency band.
However, in the case where Ag or Cu having a low resistivity is employed as the material constituting the conductor layer, since such a metal is low in melting point, in order to form a multilayered wiring board by simultaneously sintering the conductor layer and the dielectric layer, it is required to use a dielectric material that can be sintered at a low sintering temperature of from 800 to 1,050° C. As such materials that can be sintered at low temperatures and have a low specific dielectric constant and a low dielectric loss in a high-frequency band are proposed various crystallized glasses that are mainly composed of a borosilicate glass and in which crystals are crystallized at the sintering stage.
As examples of these crystallized glasses are enumerated ones in which crystals of a spinel type structure are crystallized (JP-A-9-175853), ones in which crystals of diopside are crystallized (JP-A-10-120436), and ones in which crystals of alumina, etc. are crystallized (Japanese Patent Application No. 10-320612).
Further, in order to enhance the transmission characteristics of high-frequency signals, there is proposed a multilayered wiring board in which a part of the conductor layer is constituted as a strip line or micro-strip line (for example, JP-A-11-214812). In the case where such a strip line or micro-strip line is formed, it is necessary to coat one surface of the dielectric layer substantially entirely and to form the entire electrode conductor layer, which functions as the ground conductor, as a part of the conductor layer. From the standpoint of enhancement of the transmission characteristics of high-frequency signals, it is desired that the entire electrode conductor layer is formed in a multilayered state inside the multilayered wiring board. Further, with the modulation of wiring boards, there is a demand such that the entire electrode conductor layer is formed as a capacitor electrode, thereby forming a module wiring board with a built-in capacitor.